In order to produce good quality rolls of web materials by winding (rolling up) a web and to avoid technical difficulties in further process steps (such as might be encountered by dividing the roll transversely into two segments), it is important to have a uniform winding hardness over the entire processing range of the material. Uniform hardness also tends to suppress the potential for damage, such as tears, in the web material. To achieve the desired hardness, a known method is to provide rider rolls which press against the outer circumference of the rewinding roll as the roll increases in diameter, so as to provide a constant hardness-controlling linear pressure.
In an apparatus disclosed in German Pat. No. 27 39 515 for winding, for example, a web of paper onto a core which is supported on two driven winder drums, the rewinding roll lies in a crevice (constrained space) between both of the winder drums and is pushed against by a rider roll which supplies the linear pressure to effect the desired winding hardness.
A web-winding machine is disclosed in German Pat. No. 3 102 894 for the separate winding up of lengthwise-divided web material, which has only one center drum, on which lie two rewinding rolls one on each side of a vertical plane through the axis of the center drum. In order to produce the linear pressure needed to control the winding hardness, hydraulically or pneumatically-activated cylinder drives are provided which compensate for the increasing bulk of the rewinding roll; moreover, arranged around the periphery of the rewinding roll, rider roll pairs are provided, these roll pairs being positioned in the web-winding machine at both sides of the center drum by means of swivelable levers on positioning members which can be raised or lowered.
At each winding station, the pressure exerted by means of the rider roll is maintained at least up to the point where a predetermined roll diameter is achieved, so that enough inherent firmness is achieved so as to be adequate for any further processing steps, and so that an evenly distributed winding hardness is achieved over the entire width of the rewinding roll. The pressure applied by means of each rider roll is exerted at the resting place of the common tangent of the rewinding roll, however it has been found that, disadvantageously, a pucker or air pocket can form in the web material in front of the rider roll as viewed in the direction of winding. Upon passage of such an air pocket through the roller crevice, creases are produced which reduce the quality of the wound material and result in misalignment of the wound layers. The origin of these air pockets is the entrainment of air in the crevice between the web of material and the wound roll.